This invention relates to a method of mirror-finishing the surface of a glass substrate for a magnetic disk or a liquid crystal panel for which a high level of smoothness is required.
In general, the surface of a glass substrate for a magnetic disk or the like is mirror-finished by a mechanical polishing method through direct grinding with the use of fixed or free abrading particles or by a chemical polishing method which makes use of a chemical reaction on the surface of the glass substrate which is thereafter washed and rinsed to be presented as a finished product. Mechanical polishing by means of fixed abrading particles is carried out by pressing a pad or a tape (hereinafter simply referred to as the "pad") having abrading particles affixed to its surface to the target surface of the glass substrate while supplying a coolant and causing the pad to move relative to the substrate surface. Mechanical polishing by means of free abrading particles is carried out by pressing the pad onto the target surface of the glass substrate while supplying a slurry (or a suspension containing abrading particles) and again by causing the pad to move relative to the substrate surface.
By either of these mechanical polishing methods, abrading particles with average diameter of 0.01 .mu.m-5 .mu.m are used for grinding the target surface of a glass substrate for obtaining a mirror-finished surface. At the present time, however, it is technologically very difficult to obtain abrading particles with uniform sizes and shapes, and there are always significant variations in the sizes and shapes among the abrading particles which are used. For this reason, protrusions with height differences about 100 .ANG. are usually formed on the abraded surface of a glass substrate. FIG. 4 shows the surface of a glass substrate with the maximum height of protrusion 259.4 .ANG. after it was polished by a mechanical polishing process by means of a polishing tape with a polishing layer having diamond particles with average diameter of 0.7 .mu.m affixed to its surface.
Chemical polishing, by contrast, is carried out by causing a solid phase reaction on the contact boundary surface between the abrading particles and the surface of the glass substrate to thereby generate a different substance on this contact boundary surface and then removing this portion of the contact boundary surface chemically and mechanically. Since a chemical reaction is relied upon by such a process, the deterioration or degradation of the substrate surface is extremely unlikely and fine polishing is possible since the unit area of processing is extremely small.
FIG. 5 shows a polishing machine which may be used for carrying out a chemical polishing process in a batch style in a so-called four-way operation. Disk substrates D are inserted into openings K provided in a planet gear G3 which is placed on a donut-shaped lower lapping plate J, being engaged both with an internal gear G2 on the outer (internally facing) periphery and a sun gear G1 at the center. The disk substrates D are pressed from above by an upper lapping plate (not shown) which is of the same shape as the lower lapping plate J and is provided with a plurality of openings. The chemical polishing process is carried out by supplying a slurry through these openings in the upper lapping plate, causing the upper and lower lapping plates to rotate in mutually opposite directions such that the sun gear G1 will rotated as shown by arrow W and the planet gear G3 will rotated not only around itself as shown by arrow X but also around the sun gear G1 as shown by arrow Y.
By such a chemical mechanical method of polishing by means of a double-side polishing machine, however, the abrading particles in the slurry and the glass substrate remain in a mutually contacting relationship after the polishing machine is stopped until the glass substrate is washed. Thus, a solid-phase reaction takes place on the contacting surface between the abrading particles and the glass substrate in the meantime, thereby generating unwanted substances which cannot be removed by the washing process. As a result, protrusions of hydrophilic glass with height about 100 .ANG. appears scattered over the substrate surface, as shown in FIG. 3.